I really like unsplash for background images. They're all high quality and not distracting. They also have an API that grabs a random image (of a certain size) and returns a URL to a valid image.

# i.e. calling this will give you a random image of size 1200x1080
curl https://source.unsplash.com/random/1200x1080 > img.jpg

More docs here: https://source.unsplash.com/